Magnetic recording medium containing particles with a core containing a FE16N2 phase

ABSTRACT

A magnetic recording medium comprising a nonmagnetic support and a magnetic layer formed on the support and containing a magnetic powder and a binder, wherein said magnetic powder comprises substantially spherical or ellipsoidal particles and at least one element selected from the group consisting of rare earth elements, silicon and aluminum, and has a Fe 16 N 2  phase, an average particle size of 5 to 30 nm and an axis ratio (a ratio of a major axis to a minor axis) of 1 to 2. This magnetic recording medium achieves a high output and has excellent short wavelength recording properties, since it uses a magnetic powder having a very small particle size and has a very high coercive force and a saturation magnetization suitable for high density recording.

This application is a Divisional of application Ser. No. 10/489,735filed on Mar. 16, 2004, which is now U.S. Pat. No. 7,238,439 B2 issuedJul. 3, 2007 and for which priority is claimed under 35 U.S.C. 120.Application Ser. No. 10/489,735 (U.S. Pat. No. 7,238,439) is thenational phase of PCT International Application No. PCT/JP2004/001773filed on Feb. 18, 2004 under 35 U.S.C. 371. The entire contents of eachof the above-identified applications are hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to a magnetic recording medium comprisingmagnetic powder particles having substantially spherical or ellipsoidalshapes which comprise at least iron and nitrogen as constituentelements. In particular, the present invention relates to a magneticrecording medium suitable for high density recording such as a digitalvideo tape, a backup tape for a computer, etc.

BACKGROUND ART

Coating type magnetic recording media, that is, magnetic recording mediacomprising a nonmagnetic support and a magnetic layer formed on thesupport and comprising magnetic powder and a binder, are required tohave a further increased recording density with the shift of awriting-reading system from an analog system to a digital one. Inparticular, such requirement has been increased year by year in thevideo tapes and the backup tapes for computers which are used for highdensity recording.

To cope with short wavelength recording which is inevitable to increasea recording density, it is effective to decrease the thickness of amagnetic layer to 300 nm or less, in particular, to 100 nm or less so asto decrease a thickness loss during recording. In general, a magnetoresistance head (MR head) is used as a reproducing head which is used incombination with such high density recording media.

The particle size of magnetic powder has been decreased year by year toreduce a noise. Nowadays, acicular metal magnetic powder having aparticle size of about 100 nm is practically used. Furthermore, toprevent the decrease of output caused by demagnetization during shortwavelength recording, the coercive force of the magnetic powder has beenincreased, and a coercive force of about 238.9 A/m (about 3,000 Oe) isrealized with an iron-cobalt alloy (see JP-A-3-49026, JP-A-10-83906 andJP-A-10-34085).

However, a coercive force depends on the shape of acicular magneticparticles in a magnetic recording medium comprising acicular magneticparticles. Thus, it is difficult to further decrease the particle sizeof such acicular magnetic particles. That is, if the particle size isfurther decreased, a specific surface area greatly increases andsaturation magnetization greatly decreases. Consequently, the highsaturation magnetization, which is the most significant characteristicof metal or metal alloy magnetic powder, is deteriorated, so that theuse of the metal or metal alloy becomes meaningless.

In view of the above circumstance, a magnetic recording medium using, asa magnetic powder which is totally different from the acicular magneticpowder, a rare earth element-transition metal particulate magneticpowder such as a spherical or ellipsoidal rare earth element-iron-boronmagnetic powder is proposed (see JP-A-2001-181754). This medium cangreatly decrease the particle size of the magnetic powder and achieve ahigh saturation magnetization and a high coercive force. Therefore, thismedium significantly contributes to the increase of a recording density.

Also, a magnetic recording medium using, as an iron magnetic powderhaving a non-acicular particle shape, an iron nitride magnetic powderwhich comprises random shape particles and a Fe₁₆N₂ phase as a mainphase, and has a BET specific surface area of about 10 m²/g is proposed(see JP-A-2000-277311).

However, the rare earth element-iron-boron magnetic powder ofJP-A-2001-181754 is a composite material which comes into existencebased on the balance of a high magnetic anisotropy due to the rare earthelement compound and high saturation magnetization due to the ironmaterial which forms the cores of the magnetic particles. If such amagnetic powder is further improved, for example, the coercive force ofsuch a magnetic powder is intended to be further increased, it is verydifficult to improve the magnetic properties while maintaining theoptimum dispersibility and chemical stability of the magnetic recordingmedium. JP-A-2000-277311 discloses the iron nitride magnetic powderhaving a BET specific surface area of 10 to 22 m²/g in the Example.However, such magnetic powder has a very large particle size and is notsuitable for high density magnetic recording with achieving the decreaseof noise.

The main characteristic of the iron nitride magnetic powder ofJP-A-2000-277311 is the high saturation magnetization, and the magneticpowder produced in the Examples had a saturation magnetization of 190 to200 Am²/kg (190 to 200 emu/g). However, the magnetic powder having sucha very large saturation magnetization may not be suitably used in amagnetic recording medium for high density recording, because when thesaturation magnetization is too high, the recording medium has too largesaturation magnetization so that the recording demagnetization becomesremarkable. This tendency becomes more remarkable as a recordingwavelength is made shorter. Therefore, magnetic powder ofJP-A-2000-277311 is not suitable for high density recording.

In particular, in the case of high density magnetic recording media, itis essential to adequately decrease the saturation magnetization of amagnetic powder and decrease the thickness of a magnetic layer in orderto decrease the recording demagnetization. As a magnetic flux densitydecreases, a magnetic flux from the surface of a magnetic recordingmedium deceases and in turn a reproducing output decreases. With therecent astonishing progress of magnetic head technology such as MRheads, recorded signals can be reproduced with a high sensitivity evenwith the small magnetic flux. Therefore, to achieve the high densityrecording, it is now necessary to increase a coercive force whilesetting the saturation magnetization of magnetic powder at a suitablevalue lower than a value which has been required.

SUMMARY OF THE INVENTION

In view of the above-described circumstances, one object of the presentinvention is providing a magnetic recording medium, which achieves ahigher output and has excellent short wavelength recording properties bythe use of a magnetic powder having a very small particle size, anextremely high coercive force and also a saturation magnetizationsuitable for high density recording.

To achieve the above object, the present inventors have made extensiveresearches. As a result, it has been found that an iron nitride magneticpowder, which comprises at least a Fe₁₆N₂ phase and has an substantiallyspherical or ellipsoidal particle shape and a very small particle sizesuch as an average particle size of 5 to 30 nm, has an extremely highcoercive force and a saturation magnetization suitable for high densityrecording, and that, when such a magnetic powder is used, a very thinmagnetic layer suffering from no or little decrease of output due torecording demagnetization can be realized, a high output is achieved,and excellent short wavelength recording properties are achieved.

In the present invention, an axis ratio (a ratio of a major axis to aminor axis) of the substantially spherical or ellipsoidal particles ispreferably 2 or less, more preferably from 1 to 1.5, as shown in FIG. 2.When the axis ratio exceeds 2, a part of the magnetic powder particlesmay stand in a direction across the surface of a magnetic layer in thecourse of coating a magnetic paint, so that the surface smoothness ofthe magnetic layer deteriorates and thus electromagnetic conversionproperties are worsened.

The present invention does not exclude magnetic powder particles havingsome irregularity on their surfaces, as shown in FIG. 2.

It has also been found that, when the iron nitride magnetic powdercontains at least one element selected from the group consisting of rareearth elements, silicon and aluminum, the maintenance of the shape ofthe magnetic particles in a heat-treatment step and the dispersibilityof the particles in a magnetic paint are improved, the magnetic layer isfurther made thin, and also the excellent short wavelength recordingproperties, which are hardly achieved by the prior art, can be attained.

The present invention has been completed based on the above findings.

That is, the present invention relates to a magnetic recording mediumcomprising a nonmagnetic support and a magnetic layer formed on thesupport and containing a magnetic powder and a binder, wherein saidmagnetic powder comprises substantially spherical or ellipsoidalparticles and at least one element selected from the group consisting ofrare earth elements, silicon and aluminum, and has a Fe₁₆N₂ phase, anaverage particle size of 5 to 30 nm and an axis ratio (a ratio of amajor axis to a minor axis) of 1 to 2.

In one preferred embodiment, the magnetic recording medium of thepresent invention has a coercive force of 79.6 to 318.4 kA/m (1,000 to4,000 Oe), a squareness ratio (Br/Bm) of 0.6 to 0.9 both in thelongitudinal direction, and a product (Bm·t) of a saturated magneticflux density (Bm) and a thickness (t) of a magnetic layer of 0.001 to0.1 μTm. In another preferred embodiment, the magnetic recording mediumof the present invention has at least one primer layer comprising anonmagnetic powder and a binder between the nonmagnetic layer and themagnetic layer, and a thickness of the magnetic layer of 300 nm or less,in particular, 10 to 300 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray diffraction pattern of the yttrium-iron nitridemagnetic powder used in Example 1.

FIG. 2 is a transmission electron microscopic photograph (magnification:200,000 times) of the yttrium-iron nitride magnetic powder used inExample 1.

DETAILED DESCRIPTION OF THE INVENTION

From the different viewpoint than conventional magnetic powder which isbased on shape magnetic anisotropy, the present inventors havesynthesized various magnetic powders and studied their shapes andmagnetic anisotropies to obtain a magnetic powder having improvedmagnetic properties. As a result, it has been found that a magneticpowder comprising at least iron and nitrogen as constituent elements andhaving at least a Fe₁₆N₂ phase has high magnetic anisotropy.

In the magnetic powder according to the present invention, the contentof nitrogen is, based on the iron amount, from 1.0 to 20.0 atomic %,preferably from 5.0 to 18.0 atomic %, more preferably from 8.0 to 15.0atomic %. When the content of nitrogen is too small, the smaller amountof the Fe₁₆N₂ phase is formed so that the coercive force is noteffectively increased. When the content of nitrogen is too large,nonmagnetic nitrides tend to be formed so that the coercive force is noteffectively increased, and the saturation magnetization is excessivelydecreased.

The particles of iron nitride magnetic powder of the present inventionhave substantially spherical or ellipsoidal shapes, which is differentfrom the shape of conventional acicular magnetic particles, and theiraverage particle size is preferably from 5 to 50 nm, in particular, from5 to 30 nm to satisfy the requirement for fine particles. When theparticle size is too small, the dispersibility of the magnetic powder ina magnetic paint during preparation is worsened, and the magnetic powderbecomes thermally unstable so that the coercive force tends to changeover time. When the particle size is too large, the noise increases andalso the magnetic layer may not have a smooth surface.

Herein, the average particle size is obtained by actually measuringparticle sizes of 300 particles in a photograph taken with atransmission electron microscope (TEM) and averaging the measuredparticle sizes.

The iron nitride magnetic powder of the present invention has a moderatesaturation magnetization of 80 to 160 Am²/kg (80 to 160 emu/g), which isnot excessively high, unlike the conventional magnetic powder disclosedin JP-A-2001-181754 and JP-A-2000-277311. The coercive force of the ironnitride magnetic powder of the present invention reaches 318.4 kA/m(4,000 Oe), which is about 25% higher than that of the conventionalmagnetic powders. When a magnetic recording medium is produced usingsuch a magnetic powder, it has a coercive force of 79.6 to 318.4 kA/m(1,000 to 4,000 Oe) and a squareness ratio of 0.6 to 0.9 both in thelongitudinal direction, and a product of a saturated magnetic fluxdensity and a thickness of a magnetic layer of 0.001 to 0.1 μTm.

Furthermore, when the iron nitride magnetic powder contains a rare earthelement, the surfaces of the magnetic particles are improved, and theaddition of the rare earth element is effective to improve not only themaintenance of the shape of the magnetic particles in a heat-treatmentstep but also the dispersibility of the particles in a magnetic paint.

The content of the rare earth element is usually from 0.05 to 20 atomic%, preferably from 0.1 to 15.0 atomic %, more preferably from 0.5 to10.0 atomic %, based on the amount of iron element. When the content ofthe rare earth element is too low, the dispersibility of the magneticparticles may not be sufficiently improved, and the effect to maintainthe shape of the magnetic particles in a reducing step decreases. Whenthe content of the rare earth element is too large, the ratio of theunreacted rare earth element to the rare earth element added increases,and the unreacted rare earth element interferes with the dispersing andcoating steps. Furthermore, the coercive force and saturationmagnetization may excessively decrease.

Apart from the rare earth element, the addition of silicon and/oraluminum can improve the dispersibility of the magnetic particles. Sincesilicon and aluminum are less expensive than the rare earth element,they are advantageous from the viewpoint of costs. When a rare earthelement, silicon or aluminum is used in combination, the surfaceconditions of the magnetic particles can be designed in more detail.

The substantially spherical or ellipsoidal iron nitride magnetic powder,which contains iron and nitrogen as the essential elements with thecontent of nitrogen being limited in the above specific range, and hasat least a Fe₁₆N₂ phase and a specific particle size, consists of thefiner particles than the conventional magnetic powders, and has anadequate saturation magnetization. Furthermore, the addition of the rareearth element, or silicon or aluminum can further improve thedispersibility of the magnetic particles, so that the magnetic layer canbe made thinner.

The iron nitride magnetic powder of the present invention also hasimproved storage stability. When the magnetic powder or the magneticrecording medium comprising such a magnetic powder is stored at a hightemperature and a high humidity, the magnetic characteristics such assaturated magnetic flux density are less degraded. Thus, the magneticpowder of the present invention exhibits the properties feasible for thehigh density recording magnetic recording media such as digital videotapes and backup tapes for computers coupled with the above properties.

Any reason for such effects has not been clarified, but some reason maybe as follows:

The magnetic powder of the present invention may have the peculiarproperties, which are not found in the conventional magnetic powders,through the synergistic effect of the high magnetic anisotropy of theFe₁₆N₂ phase and the surface magnetic anisotropy of the fine particles.Furthermore, when the compound comprising the rare earth element,silicon or aluminum is present on the surfaces of the magneticparticles, the dispersibility of the magnetic powder is improved, andsuch elements contribute to the maintenance of the shape of the magneticparticles in the reducing step so that the particle size distribution ismade narrow.

Although the Fe₁₆N₂ phase may be present inside the particles of theiron nitride magnetic powder of the present invention, the largerportion of the Fe₁₆N₂ phase is present preferably in the outer layers ofthe magnetic particles to increase the surface magnetic anisotropy.Thereby, the intended effects are more easily achieved.

To achieve the high dispersibility and good maintenance of the shape ofthe magnetic particles, the rare earth element or silicon or aluminum isalso present in the form of the compound of such an element in the outerlayers of the magnetic particles, which comprise an inner layer and anouter layer, although it may be present inside the magnetic particles.

In the present invention, the inner layer of the magnetic particlespreferably consists of the Fe₁₆N₂ phase, while it may comprise a mixedphase of the Fe₁₆N₂ phase and an α-Fe phase. Such a mixed phase canprovide flexibility to the design of the magnetic recording media, sincethe desired coercive force can easily be attained by the adjustment ofthe ratio of the Fe₁₆N₂ phase to the α-Fe phase.

Examples of the rare earth element include yttrium, ytterbium, cesium,praseodymium, lanthanum, europium, neodymium, etc. Among them, yttrium,samarium or neodymium is preferably used, since these elements have alarge effect to maintain the shape of the magnetic particles in thereducing step.

Apart from silicon and aluminum, the magnetic powder of the presentinvention may optionally comprise boron, phosphorus, carbon, calcium,magnesium, zirconium, barium, strontium, etc. as an effective element.When such an element is used together with the rare earth element, themagnetic powder has the high shape maintenance and good dispersibility.

Now, the production method of the iron nitride magnetic powder isexplained.

As a raw material, an oxide or hydroxide of iron is used. Examples ofsuch oxide or hydroxide include hematite, magnetite, goethite, etc. Theaverage particle size of the raw material is not limited, and is usuallyfrom 5 to 80 nm, preferably from 5 to 50 nm, more preferably from 5 to30 nm. When the particle size of the raw material is too small, theparticles tend to be sintered together in the reducing treatment. Whenit is too large, the particles may be less uniformly reduced so that thecontrol of the particle size and/or magnetic properties of the magneticpowder is difficult.

The rare earth element may be adhered to the surface of the raw materialparticles. Usually, the raw material is dispersed in an aqueous solutionof an alkali or an acid. Then, the salt of the rare earth element isdissolved in the solution and the hydroxide or hydrate of the rare earthelement is precipitated and deposited on the raw material particles by aneutralization reaction, etc.

Alternatively, a compound of silicon or aluminum may be dissolved in asolvent and the raw material is dipped in the solution so that such anelement can be deposited on the raw material particles. To effectivelycarry out the deposition of such an element, an additive such as areducing agent, a pH-buffer, a particle size-controlling agent, etc. maybe mixed in the solution. Silicon or aluminum may be deposited at thesame time as, or alternately with the deposition of the rare earthelement.

Then, the raw material particles are reduced by heating them in theatmosphere of a reducing gas. The kind of the reducing gas is notlimited. Usually a hydrogen gas is used, but other reducing gas such ascarbon monoxide may be used.

A reducing temperature is preferably from 300 to 600° C. When thereducing temperature is lower than 300° C., the reducing reaction maynot sufficiently proceed. When the reducing temperature exceeds 600° C.,the particles tend to be sintered.

After the thermal reduction of the particles, they are subjected to anitriding treatment. Thereby, the magnetic powder comprising iron andnitrogen as the essential element according to the present invention isobtained. The nitriding treatment is preferably carried out with a gascontaining ammonia. Apart from pure ammonia gas, a mixture of ammoniaand a carrier gas (e.g. hydrogen gas, helium gas, nitrogen gas, argongas, etc.) may be used. The nitrogen gas is preferable since it isinexpensive.

The nitriding temperature is preferably from 100 to 300° C. When thenitriding temperature is too low, the particles are not sufficientlynitrided so that the coercive force may insufficiently be increased.When the nitriding temperature is too high, the particles areexcessively nitrided so that the proportion of Fe₄N and Fe₃N phasesincreases and thus the coercive force may rather be decreased and alsothe saturation magnetization tends to excessively decrease.

The nitriding conditions are selected so that the content of thenitrogen atoms is usually from 1.0 to 20.0 atomic % based on the amountof iron in the magnetic powder obtained. When the content of thenitrogen atoms is too small, the coercive force is not effectivelyincreased since the generated amount of the Fe₁₆N₂ phase is small. Whenthe content of the nitrogen atoms is too large, Fe₄N and Fe₃N phasestend to form and thus the coercive force may rather be decreased andalso the saturation magnetization tends to excessively decrease.

Different from the conventional acicular magnetic powders the magnetismof which is based on the shape magnetic anisotropy, the iron nitridemagnetic powder of the present invention has the large crystallinemagnetic anisotropy. Thus, when the particles of the magnetic powderhave the substantially spherical shape, they may exhibit the largecoercive force in one direction.

When the magnetic powder of the present invention comprises fineparticles having an average particle size of 5 to 30 nm, it has a highcoercive force and an adequate saturation magnetization, which enablethe recording and erasing with a magnetic head. Therefore, it canprovide excellent electromagnetic conversion properties to a coatingtype magnetic recording medium having a thin magnetic layer.Accordingly, the magnetic powder of the present invention has thesaturation magnetization, coercive force, particle size and particleshape, all of which essentially serve for the formation of a thinmagnetic layer.

The present invention achieves the good recording-reproducingcharacteristics by the formation of a thin magnetic layer having, forexample, 300 nm or less using the specific magnetic powder describedabove. The present invention involves a breakthrough in technologywherein a magnetic material has been found which has a high crystallinemagnetic anisotropy, that is, a high coercive force and an adequatesaturation magnetization, and also good dispersibility, while which hasa very small particle size and the spherical or ellipsoidal shape. Sucha magnetic material breaks the bounds of common knowledge in the fieldof the coating type magnetic recording media.

The magnetic recording medium of the present invention can be producedby applying a magnetic paint, which is prepared by mixing the ironnitride magnetic powder and a binder in a solvent, on the surface of anonmagnetic support and drying the applied paint to form a magneticlayer. Prior to the formation of the magnetic layer, a primer layer maybe formed on the surface of the nonmagnetic support by applying a primercoating containing nonmagnetic powder (e.g. iron oxide, titanium oxide,aluminum oxide, etc.) and a binder and drying it, and then the magneticlayer is formed on the primer layer.

Hereinafter, the elements of the magnetic recording medium of thepresent invention, that is, (a) nonmagnetic supports, (b) a magneticlayer, and (c) a primer layer will be explained. Furthermore, (d)binders and (e) lubricants, which are contained in the magnetic layerand/or the primer layer, will be explained.

When the magnetic recording medium is a magnetic tape, (f) a backcoatlayer is preferably formed on the back face of the nonmagnetic supportopposite to the surface having the magnetic layer thereon by applying abackcoat paint to the back face and drying it. Thus, the backcoat layer(f) will also be explained.

In addition, (g) solvents contained in the magnetic paint, primer paintand backcoat paint, (h) methods for dispersing and applying such paints,and (i) the LRT treatment of the magnetic layer, which are employed inthe production of the magnetic recording medium, will be explained.

(a) Nonmagnetic Supports

The nonmagnetic support may be any one of those conventionally used inthe magnetic recording media. Specific examples of the support areplastic films having a thickness of 2 to 15 μm, preferably 2 to 7 μm,made of polyesters (e.g. polyethylene terephthalate, polyethylenenaphthalate, etc.), polyolefin, cellulose triacetate, polycarbonate,polyamide, polyimide, polyamideimide, polysulfone, aramide, aromaticpolyamide, etc.), and the like. When the thickness of the nonmagneticsupport is less than 2 μm, it is difficult to prepare such a thin film,or the tape strength decreases. When the thickness exceeds 7 μm, thetotal thickness of the magnetic tape increases so that the memorycapacity per one reel decreases.

In the case of a magnetic tape, a nonmagnetic support having theanisotropy of Young's moduli is used. The Young's modulus of thenonmagnetic support in its machine direction varies with the thicknessof the support and is preferably at least 4.9 GPa (500 kg/mm²). When thethickness of the nonmagnetic support is 5 μm or less, the Young'smodulus is preferably at least 9.8 GPa (1,000 kg/mm²). When the Young'smodulus is too small, the strength of the magnetic tape is insufficient,or the tape running becomes unstable.

A ratio of a Young's modulus in the machine direction (MD) to that (TD)in the transverse direction of the substrate (MD/TD) is preferably from0.60 to 0.80 in a helical scan type. When this ratio (MD/TD) is withinthis range, the flatness of the outputs from an entrance to an exit of atrack of a magnetic head increases, although no mechanism thereof hasbeen clarified. In the case of a linear track system, the MD/TD ratio ispreferably from 1.0 to 1.8, more preferably from 1.1 to 1.7. When theMD/TD ratio is within this range, the head touch is improved. Examplesof such a nonmagnetic support include a polyethylene terephthalate film,a polyethylene naphthalate film, an aromatic polyamide film, and anaromatic polyimide film, etc.

(b) Magnetic Layer

The thickness of the magnetic layer is 300 nm or less to solve theproblem of output decrease caused by demagnetization, which is anessential problem in the longitudinal recording. The thickness of themagnetic layer is determined in connection with there cording wavelength used. The effects of the present invention are remarkable, whenthe present invention is employed with a system having the shortestrecording wavelength of 1.0 μm or less.

The thickness of the magnetic layer is 300 nm or less, preferably from10 to 300 nm, more preferably from 10 to 250 nm, most preferably from 10to 200 nm. When the thickness of the magnetic layer exceeds 300 nm, thereproducing output decreases due to the thickness loss, and the productof the residual magnetic flux density and the thickness of the magneticlayer is too large so that the reproduction output may be skewed by thesaturation of the MR head. When the thickness of the magnetic layer isless than 10 nm, a uniform magnetic layer may not be formed.

Since the magnetic powder of the present invention has a very smallaverage particle size of 5 to 30 nm, and the substantially spherical orellipsoidal shape, it enables the formation of a very thin magneticlayer, which cannot be formed with the conventional acicular magneticpowder.

In the case of a magnetic tape, the coercive force of the magnetic layerin the longitudinal direction is usually from 79.6 to 318.4 kA/m (1,000to 4,000 Oe), preferably from 119.4 to 318.4 kA/m (1,500 to 4,000 Oe).When the coercive force is less than 79.6 kA/m, the output tends to belowered due to the demagnetizing field demagnetization, as the recordingwavelength is shortened. When the coercive force exceeds 318.4 kA/m, itis difficult to record signals with a magnetic head.

The squareness ratio (Br/Bm) in the longitudinal direction of themagnetic tape is usually from 0.6 to 0.9, in particular, from 0.8 to0.9.

Furthermore, the product of the saturated magnetic flux density and thethickness is usually from 0.001 to 0.1 μTm, preferably from 0.0015 to0.05 μTm. When the product is less than 0.001 μTm, the reproducingoutput is low even if a MR head is used. When the product exceeds 0.1μTm, the high output may not be attained in the intended shortwavelength range.

In the case using a MR head, the contact of the magnetic recordingmedium to the MR head is improved, and the reproducing output increases,when the average surface roughness Ra of the magnetic layer is from 1.0to 3.2 nm, and (P₁-P₀) is from 10 to 30 nm and (P₁-P₂₀) is 5 nm or less,wherein P₀ is the center value of the unevenness of the magnetic layer;P₁ is the height of the highest projection of the magnetic layer; andP₂₀ is the height of the 20th highest projection. The evaluationconditions are described in JP-A-2002-203308 (or US-2002-0164503-A).

The magnetic layer may contain conventional carbon black to improve theconductivity and the surface lubricity. As carbon black, acetyleneblack, furnace black, thermal black, etc. may be used. Carbon blackhaving a particle size of 5 to 200 nm is preferably used, and carbonblack having a particle size of 10 to 100 nm is more preferable. Whenthe particle size of carbon black is less than 5 nm, the dispersion ofcarbon black particles is difficult. When the particle size of carbonblack exceeds 200 nm, a large amount of carbon black should be added. Ineither case, the surface of the magnetic layer is roughened and thus theoutput tends to decrease.

The amount of carbon black is preferably from 0.2 to 5% by weight, morepreferably from 0.5 to 4% by weight, based on the weight of theferromagnetic powder. When the amount of carbon black is less than 0.2%by weight, the effect of the addition of carbon black is insufficient.When the amount of carbon black exceeds 5% by weight, the surface of themagnetic layer tends to be roughened.

(c) Primer Layer

A primer layer is not an essential element, but may be formed betweenthe nonmagnetic support and the magnetic layer to improve the durabilityof the magnetic recording medium. The thickness of the primer layer ispreferably from 0.1 to 3.0 μm, more preferably from 0.15 to 2.5 μm.

When the thickness of the primer layer is less than 0.1 μm, thedurability of the magnetic tape may deteriorate. When the thickness ofthe primer layer exceeds 3.0 μm, the effect to improve the durability ofthe magnetic tape is saturated, and the total thickness of the magnetictape increases. Accordingly, the length of the tape per one reeldecreases and, in turn, the recording capacity decreases.

The primer layer may contain nonmagnetic powder such as titanium oxide,iron oxide or aluminum oxide to control the viscosity of the primerpaint or the stiffness of the medium.

Preferably, the nonmagnetic iron oxide is used as such or in combinationwith aluminum oxide.

The content of the nonmagnetic iron oxide is preferably from 35 to 83%by weight, more preferably from 40 to 80% by weight. When the content ofthe nonmagnetic iron oxide is less than 35% by weight, the effect toimprove the strength of the coated film is insufficient. When thiscontent exceeds 83% by weight, the strength of the coated film maydecrease instead. The content of aluminum oxide is usually from 0 to 20%by weight, preferably from 2 to 10% by weight.

The nonmagnetic iron oxide particles may be of a needle shape, orparticulate or random shape. The needle shape iron oxide preferably hasan average major axis length of 50 to 200 nm and an average minor axislength (average particle size) of 5 to 50 nm. The particulate or randomshape nonmagnetic iron oxide has an average particle size is preferablyfrom 5 to 200 nm, more preferably from 5 to 150 nm, most preferably from5 to 100 nm. When the particle size is smaller than the above lowerlimit, the iron oxide powder may not be uniformly dispersed in asolvent. When the particle size is larger than the above upper limit,the irregularity at the interface between the primer layer and themagnetic layer tends to increase.

Aluminum oxide preferably has an average particle size of 10 to 100 nm,more preferably from 20 to 100 nm, most preferably from 30 to 100 nm.When the average particle size is less than the above lower limit,aluminum oxide powder may not be uniformly dispersed in a solvent. Whenthe particle size is larger than the above upper limit, the irregularityat the interface between the primer layer and the magnetic layer tendsto increase.

The primer layer may contain carbon black such as acetylene black,furnace black, thermal black, etc. to improve the conductivity.

The carbon black preferably has an average particle size of 5 to 200 nm,more preferably 10 to 100 nm. Since the carbon black has a texture, thedispersion of the carbon black is difficult when the average particlesize is too small, while the surface smoothness of the primer layerdecreases when the average particle size is too large.

The content of carbon black depends on the particle size of the carbonblack, and is preferably from 15 to 35% by weight. When the content ofcarbon black is less than 15% by weight, the effect to improve theconductivity is insufficient. When the content of the carbon blackexceeds 35% by weight, such effect is saturated.

Preferably, the primer layer contains 15 to 35% by weight of carbonblack having a particle size of 15 to 80 nm, more preferably 20 to 30%by weight of carbon black having a particle size of 20 to 50 nm. Whencarbon black having such a particle size is used in such an amount, theelectrical resistance decreases and the running irregularity issuppressed.

The present invention does not avoid the addition of large size carbonblack having an average particle size beyond the above range, as long asthe surface smoothness is not impaired. In this case, preferably, thesum of the small size carbon black and the large size carbon black iswithin the above range.

The surface roughness of the magnetic layer, which is formed on theprimer layer by a wet-on-wet method, can be reduced, when the primerlayer contains 15 to 35% by weight of carbon black having an averageparticle size of 10 to 100 nm, 35 to 83% by weight of nonmagnetic ironoxide having an average major axis length of 50 to 200 nm and an averageminor axis length of 5 to 50 nm, and optionally 0 to 20% by weight ofaluminum oxide having an average particle size of 10 to 100 nm, based onthe weight of the total nonmagnetic particles contained in the primerlayer.

(d) Binders

A binder to be contained in the primer layer or the magnetic layer maybe a combination of a polyurethane resin and at least one resin selectedfrom the group consisting of vinyl chloride base reins such as a vinylchloride resin, a vinyl chloride-vinyl acetate copolymer resin, a vinylchloride-vinyl alcohol copolymer resin, a vinyl chloride-vinylacetate-vinyl alcohol copolymer resin, a vinyl chloride-vinylacetate-maleic anhydride copolymer resin and a vinyl chloride-hydroxylgroup-containing alkyl acrylate copolymer resin; nitrocellulose; anepoxy resin; and the like.

Among them, a mixture of a vinyl chloride base resin and a polyurethaneresin is preferable. In particular, a mixture of a vinylchloride-hydroxyl group-containing alkyl acrylate copolymer resin and apolyurethane resin is preferably used.

Examples of the polyurethane resin include polyesterpolyurethane,polyetherpolyurethane, polyetherpolyesterpolyurethane,polycarbonatepolyurethane, polyesterpolycarbonate-polyurethane, etc.

Preferably, a binder resin having a functional group is used to increasethe dispersibility and filling rate of the magnetic powder, etc.Examples of the functional group are —COOM, —SO₃M, —OSO₂M, —P═O(OM)₃,—O—P═O(OM)₂ [wherein M is a hydrogen atom, an alkali metal or an aminesalt], —OH, —NR₁R₂, —NR₃R₄R₅ [wherein R₁, R₂, R₃, R₄ and R₅ are each ahydrogen atom or a hydrocarbon group having 1 to 10 carbon atoms], or anepoxy group. When two or more resins are used in combination, it ispreferable that the polarities of the functional groups of the resinsare the same. In particular, the combination of resins both having —SO₃Mgroups is preferable.

The binder is used in an amount of 7 to 50 parts by weight, preferablyfrom 10 to 35 parts by weight, based on 100 parts by weight of the solidpowders such as the magnetic powder and nonmagnetic powder. Inparticular, the combination of 5 to 30 parts by weight of a vinylchloride base resin and 2 to 20 parts by weight of the polyurethaneresin is preferably used as binders.

It is preferable to use a thermally curable crosslinking agent, whichbonds with the functional groups in the binder to crosslink the binder.As the crosslinking agent, the following are preferably used: tolylenediisocyanate, hexamethylene diisocyanate and isophorone diisocyanate;reaction products of these isocyanates with compounds having pluralhydroxyl groups such as trimethylolpropane; condensation products ofthese isocyanates, and the like. The crosslinking agent is used in anamount of 10 to 50 parts by weight, preferably 10 to 35 parts by weight,based on 100 parts by weight of the binder.

When the amount of the crosslinking agent to be contained in themagnetic layer is from 30 to 60% by weight of the total weight of thecrosslinking agents contained in the primer layer and the magneticlayer, in particular, about a half of the weight of the crosslinkingagent contained in the primer layer, the coefficient of dynamic frictionof the magnetic layer against the slider of the MR head is preferablydecreased. When the amount of the crosslinking agent is less than 30% byweight, the film strength of the magnetic layer tends to decrease,while, when it exceeds 60% by weight, the severer conditions for thewiping treatment using tissue (the LRT treatment conditions describedbelow) should be used so as to decrease the coefficient of dynamicfriction against the slider, which leads to the increase of costs.

(e) Lubricants

As a lubricant to be contained in the primer layer or the magneticlayer, any one of conventionally used fatty acids, fatty acid esters andfatty acid amides may be used. Among them, the combination of a fattyacid having at least 10 carbon atoms, preferably 12 to 30 carbon atomsand a fatty acid ester having a melting point of 35° C. or less,preferably 10° C. or less is particularly preferable.

The fatty acid having at least 10 carbon atoms may be linear or branchedone, or a cis- or trans-isomer. The linear fatty acid is preferablebecause of good lubrication properties. Examples of such fatty acidsinclude lauric acid, myristic acid, stearic acid, palmitic acid, behenicacid, oleic acid, linoleic acid, etc. Among them, myristic acid, stearicacid, palmitic acid, etc. are preferable.

Examples of the fatty acid esters having a melting point of 35° C. orless include n-butyl oleate, hexyl oleate, n-octyl oleate, 2-ethylhexyloleate, oleyl oleate, n-butyl laurate, heptyl laurate n-butyl myristate,n-butoxyethyl oleate, trimethylolpropane trioleate, n-butyl stearate,sec-butyl stearate, isoamyl stearate, butylcellosolve stearate, etc.These fatty acid esters may be used in combination since the strength ofcoated films and an exuded amount thereof can be controlled depending onthe difference of the molecular weights and chemical structures, and thedifference of the melting points. Since those esters have the abovemelting points, they can easily migrate to the surface of the magneticlayer and thus they effectively exhibit the lubrication function, whenthe magnetic layer and the magnetic head are in contact with each otherat a high speed, even if they are exposed to a low temperature and a lowhumidity.

In the case of a magnetic tape having a primer layer, the coating layercomprising the primer layer and the magnetic layer preferably containlubricants having different functions.

The coefficient of dynamic friction of the magnetic tape against theguide of a tape-running system or the like can be decreased, when theprimer layer contains 0.5 to 4% by weight of a higher fatty acid and 0.2to 3% by weight of a higher fatty acid ester, based on the weight of theentire powder components in the primer layer. When the amount of thehigher fatty acid is less than 0.5% by weight, the effect to decreasethe coefficient of dynamic friction is insufficient. When the amount ofthe higher fatty acid exceeds 4% by weight, the primer layer may beplasticized and thus the toughness of the primer layer may be lost. Whenthe amount of the higher fatty acid ester is less than 0.5% by weight,the effect to decrease the coefficient of friction is insufficient. Whenthe amount of the higher fatty acid ester exceeds 3% by weight, theamount of the higher fatty acid ester which migrates to the magneticlayer may become too large, so that the magnetic tape may stick to theguide or the like of the feeding system.

The coefficient of dynamic friction of the magnetic tape against theguide roller of the feeding system or the slider of the MR head can bedecreased, when the magnetic layer contains 0.2 to 3% by weight of afatty acid amide (e.g. an amide of a higher fatty acid such as palmiticacid, stearic acid, etc.), and 0.2 to 3% by weight of a higher fattyacid ester, based on the weight of the magnetic powder. When the amountof the fatty acid amide is less than 0.2% by weight, the coefficient of(dynamic) friction between the head slider and the magnetic layer tendsto increase. When the amount of the fatty acid amide exceeds 3% byweight, the fatty acid amide bleeds out and causes some defects such asdropout. When the amount of the higher fatty acid ester is less than0.2% by weight, the coefficient of dynamic friction is hardly decreased.When the amount of the higher fatty acid ester exceeds 3% by weight, themagnetic tape sticks to the guide of the feeding system.

The intermigration of the lubricants between the magnetic layer and theprimer layer may be allowed.

When the MR head is used, the coefficient of dynamic friction (μ_(Msl))between the magnetic layer and the slider of the MR head is preferably0.30 or less, more preferably 0.25 or less. When this coefficient ofdynamic friction exceeds 0.30, the spacing loss tends to arise due tothe contamination of the slider. The coefficient of dynamic friction ofless than 0.10 is hardly realized.

The coefficient of dynamic friction (μ_(Msus)) between the magneticlayer and SUS is preferably from 0.10 to 0.25, more preferably from 0.12to 0.20. When this coefficient of dynamic friction is less than 0.10,the tape is so slidable on the guide portion that the tape cannot runstably. When this coefficient of dynamic friction exceeds 0.25, theguide rollers may easily be contaminated. The ratio of[(μ_(Msl))/(μ_(Msus))] is preferably from 0.7 to 1.3, more preferablyfrom 0.8 to 1.2. In this preferred range, dislocation from a track(off-track) caused by the tape-meandering becomes smaller.

(f) Backcoat Layer

A backcoat layer is not an essential element but is preferably formed onthe back face of the nonmagnetic support opposite to the surfacecarrying the magnetic layer thereon.

The thickness of a backcoat layer is preferably from 0.2 to 0.8 μm, morepreferably from 0.3 to 0.8 μm, particularly from 0.3 to 0.6 μm. When thethickness of the backcoat layer is less than 0.2 μm, the effect toincrease the running property is insufficient. When the thickness of thebackcoat layer exceeds 0.8 μm, the total thickness of the magnetic tapeincreases, so that the recording capacity of the tape per one reeldecreases.

The center line average surface roughness Ra of the backcoat layer ispreferably from 3 to 15 nm, more preferably from 4 to 10 nm.

The coefficient of dynamic friction (μ_(Bsus)) between the backcoatlayer and SUS (stainless steel) is preferably from 0.10 to 0.30, morepreferably from 0.10 to 0.25. When this coefficient of dynamic frictionis less than 0.10, the magnetic tape becomes excessively slidable on theguide rollers, so that the running of the tape becomes unstable. Whenthis coefficient of dynamic friction exceeds 0.30, the guide rollerstend to be contaminated. The ratio of [(μ_(Msl))/(μ_(Bsus))] ispreferably from 0.8 to 1.5 more preferably from 0.9 to 1.4. Within thisrange, dislocation from a track (off-track) on the magnetic tape due tothe tape-meandering becomes smaller.

The backcoat layer may contain carbon black such as acetylene black,furnace black, thermal black, etc. In general, carbon black with a smallparticle size and carbon black with a large particle size are usedtogether. The total amount of carbon black with a small particle sizeand carbon black with a large particle size is preferably from 60 to 98%by weight, more preferably from 70 to 95% by weight.

The average particle size of small particle size carbon black is usuallyfrom 5 to 100 nm, preferably from 10 to 100 nm. When the averageparticle size of small particle size carbon black is too small, thedispersion thereof is difficult. When the average particle size of smallparticle size carbon black too large, a large amount of carbon blackshould be added. In either case, the surface of the backcoat layer isroughened so that the magnetic layer may be embossed with such aroughened surface of the backcoat layer.

When the large particle size carbon black having an average particlesize of 300 to 400 nm is used in an amount of 5 to 15% by weight of theamount of carbon black of small particle size, the surface of thebackcoat layer is not roughened and the effect to increase thetape-running performance is improved.

To enhance the strength of the backcoat layer, it is preferable to addadditives which are usually added to the backcoat layer, for example,iron oxide or aluminum oxide each having an average particle size of0.05 to 0.6 μm, more preferably 0.07 to 0.4 μm, most preferably 0.07 to0.35 μm to the backcoat layer. When the average particle size of theadditives is less than 0.05 μm, the effect to increase the strength isinsufficient. When the average particle size of the additives exceeds0.6 μm, the surface roughness of the backcoat layer increases so thatthe magnetic layer may be embossed with the roughened surface of thebackcoat layer.

The additive such as the iron oxide above is preferably used in anamount of 2 to 40% by weight, more preferably 2 to 30% by weight,particularly preferably 2 to 20% by weight, most preferably 5 to 15% byweight. When the amount of the additive is less than 2% by weight, theeffect to improve the strength is low. When the amount of the additiveexceeds 40% by weight, the surface roughness of the backcoat layerincreases.

Usually, the iron oxide or the aluminum oxide is added singly. When ironoxide and aluminum oxide are added together, the amount of aluminumoxide is preferably 20% by weight of less of the weight of the ironoxide.

A binder to be contained in the backcoat layer may comprise the sameresin as used in the magnetic layer and the primer layer. Among theseresins, the combination of a cellulose resin and a polyurethane resin ispreferably used so as to decrease the coefficient of friction and toimprove the tape-running performance.

The amount of the binder in the backcoat layer is usually from 40 to 150parts by weight, preferably from 50 to 120 parts by weight, morepreferably from 60 to 110 parts by weight, particularly from 70 to 110parts by weight, based on the total 100 parts by weight of carbon blackand the inorganic nonmagnetic powder in the backcoat layer. When theamount of the binder is too small, the strength of the backcoat layer isinsufficient. When the amount of the binder too large, the coefficientof friction may increase. Preferably, 30 to 70 parts by weight of acellulose resin and 20 to 50 parts by weight of a polyurethane resin areused.

To cure the binder, a crosslinking agent is preferably used. Thecrosslinking agent to be contained in the backcoat layer may be the sameas those used in the magnetic layer and the primer layer, for example, apolyisocyanate compound. The amount of the crosslinking agent is usuallyfrom 10 to 50 parts by weight, preferably from 10 to 35 parts by weight,more preferably from 10 to 30 parts by weight, based on 100 parts byweight of the binder. When the amount of the crosslinking agent is toosmall, the film strength of the backcoat layer tends to decrease. Whenthe amount of the crosslinking agent too large, the coefficient ofdynamic friction of the backcoat layer against SUS increases.

(g) Solvents of Paints

To prepare the magnetic paint, primer paint and backcoat paint, anyconventionally used organic solvent may be used as a solvent. Examplesof the organic solvent include aromatic solvents (e.g. benzene, toluene,xylene, etc.), ketone solvents (e.g. acetone, cyclohexanone, methylethyl ketone, methyl isobutyl ketone, etc.), ester solvents (e.g. ethylacetate, butyl acetate, etc.), carbonate solvents (e.g. dimethylcarbonate, diethyl carbonate, etc.), and alcohols (e.g. ethanol,isopropanol, etc.). In addition, other organic solvents such as hexane,tetrahydrofuran, dimethylformamide, etc. may be used.

(h) Dispersion and Coating of Paints

In the preparation of the magnetic paint, primer paint and backcoatpaint, any known method for the preparation of paints can be used. Inparticular, a kneading process using a kneader or the like and a primarydispersion process are preferably used in combination. In the primarydispersion process, a sand mill is preferably used since thedispersibility of the magnetic powder is improved and also the surfaceproperties of the layers can be controlled.

When the magnetic paint, primer paint or backcoat paint is applied onthe nonmagnetic support, any conventional application methods such asgravure coating, roll coating, blade coating, extrusion coating, etc.may be used. The application method of the primer layer and the magneticlayer may be a sequential multiple layer coating method in which themagnetic paint of the magnetic layer is applied on the primer layerwhich has been applied on the nonmagnetic support and dried, or asimultaneous multiple layer coating method (wet-on-wet method) in whichthe primer layer and the magnetic layer are applied at the same time.

In view of the leveling of the thin magnetic layer in the course of theapplication, the simultaneous multiple layer coating method, whichapplies the paint for the magnetic layer while the primer layer is stillwet, is preferably used.

(i) LRT (Lapping/Rotary/Tissue) Treatment

The magnetic layer is subjected to a LRT treatment so as to optimize thesurface smoothness, the coefficient of friction against the slider ofthe MR head and the cylinder material, the surface roughness and thesurface shape. Thus, the running performance of the magnetic tape andthe reproducing output with the MR head are improved, and the spacingloss is reduced.

The LRT treatment includes (A) lapping, (B) rotary treatment and (C)Tissue treatment as explained below.

(A) Lapping:

An abrasive tape (lapping tape) is moved by a rotary roll at a constantrate (standard: 14.4 cm/min.) in a direction reverse to the tape-feedingdirection (standard: 400 m/min.), and is allowed to contact with thesurface of the magnetic layer of the magnetic tape while being pressedunder the guide block. In this step, the magnetic layer is polishedwhile the unwinding tension of the magnetic tape and the tension of thelapping tape being maintained constant (standard: 100 g and 250 g,respectively).

The abrasive tape used in this step may be an abrasive tape with fineabrasive particles such as M20000, WA10000 or K10000. It is possible touse an abrasive wheel (lapping wheel) in place of or in combination withthe abrasive tape. In case where frequent replacement is needed, theabrasive tape alone is used.

(B) Rotary Treatment

A rotary wheel having air-bleeding grooves (standard: width of 1 inch(25.4 mm); diameter of 60 mmφ; air-bleeding groove width of 2 mm; grooveangle of 45 degrees, manufactured by KYOWA SEIKO Co., Ltd.) is rotatedat a constant revolution rate (usually 200 to 3,000 rpm; standard: 1,100rpm) in a direction reverse to the feeding direction of the magneticlayer, while being allowed to be in contact with the magnetic layer ofthe magnetic tape at a constant contact angle (standard: 90 degrees).Thus, the surface of the magnetic layer is treated.

(C) Tissue Treatment

Tissue (a woven fabric, for example, Toraysee manufactured by Toray) isfed at a constant rate (standard: 14.0 mm/min.) by rotary rods, in adirection reverse to the feeding direction of the magnetic tape, so asto clean the surfaces of the backcoat layer and the magnetic layer ofthe magnetic tape, respectively.

EXAMPLES

The present invention will be explained in detail by way of thefollowing Examples. In Examples and Comparative Examples, “parts”¹ are“parts by weight”.

Example 1 (A) Preparation of Iron Nitride Magnetic Powder

Magnetite particles having a substantially spherical shape and anaverage particle size of 25 nm (10 g) was dispersed in water (500 cc)with an ultrasonic disperser for 30 minutes. To this dispersion, yttriumnitrate (2.5 g) was added and dissolved followed by stirring for 30minutes.

Separately, sodium hydroxide (0.8 g) was dissolved in water (100 cc).This aqueous solution of sodium hydroxide was dropwise added to theabove dispersion over about 30 minutes. After the addition of theaqueous solution, the mixture was stirred for further 1 hour. Thereby,yttrium hydroxide was deposited on the surfaces of the magnetiteparticles. Then, the magnetite particles was washed with water andfiltrated. The recovered particles were dried at 90° C. to obtain apowder comprising the magnetite particles the surfaces of which werecovered with yttrium hydroxide.

The powder comprising the magnetite particles the surfaces of which werecovered with yttrium hydroxide was heated and reduced in a hydrogenstream at 450° C. for 2 hours to obtain an yttrium-iron magnetic powder.This powder was cooled to 150° C. over about 1 hour while flowinghydrogen gas, and then the hydrogen gas was switched to an ammonia gas,and the particles were nitrided for 30 hours while maintaining thetemperature at 150° C. Thereafter, the particles were cooled from 150°C. to 90° C. while flowing the ammonia gas, and then the ammonia gas wasswitched to a mixed gas of oxygen and nitrogen to stabilize theparticles for 2 hours.

After that, the particles were further cooled from 90° C. to 40° C. andmaintained at 40° C. for about 10 hours, while flowing the mixed gas ofoxygen and nitrogen, and then they were recovered in an air.

The contents of yttrium and nitrogen of the resulting yttrium-ironnitride magnetic powder were analyzed with a fluorescent X-ray analysisand 5.3 atomic % and 10.8 atomic %, respectively, based on the amount ofiron. The X-ray diffraction pattern showed the profile corresponding tothe Fe₁₆N₂ phase. FIG. 1 is the X-ray diffraction pattern of theyttrium-iron nitride magnetic powder of this Example, which shows thediffraction peaks assigned to the Fe₁₆N₂ phase and the α-Fe phase. Thus,it was confirmed that the yttrium-iron nitride magnetic powder of thisExample composed of the mixed phase of the Fe₁₆N₂ phase and the α-Fephase.

Furthermore, the particles were observed with a high dissolutiontransmission electron microscope. The particle shape was substantiallyspherical, and the average particle size was 22 nm. FIG. 2 shows thephotograph (magnification: 200,000 times) of the magnetic powder of thisExample taken with the transmission electron microscope. The magneticpowder had a BET specific surface area of 53.2 m²/g.

The saturation magnetization and coercive force of the magnetic powderof this Example, which were measured by applying a magnetic field of1,270 kA/m (16 kOe), were 135.2 Am²/kg (135.2 emu/g) and 226.9 kA/m(2,850 Oe), respectively.

(B) Production of Magnetic Tape

The following components of a primer paint were kneaded with a kneaderand then dispersed with a sand mill at a residence time of 60 minutes.To the dispersion, 6 parts of a polyisocyanate was added, and themixture was stirred and filtrated to obtain a primer paint.

Separately, the following components (1) of a magnetic paint werekneaded with a kneader and dispersed with a sand mill at a residencetime of 45 minutes. To the resulting mixture, the following components(2) of a magnetic paint were added and mixed. Thereafter, the mixturewas stirred in a vessel in which a radially magnetized cylindricalpermanent magnet was inserted to obtain a magnetic paint.

<Components of a Primer Paint>

Iron oxide powder (av. particle size: 55 nm) 70 parts Aluminum oxidepowder (av. particle size: 80 nm) 10 parts Carbon black (av. particlesize: 25 nm) 20 parts Vinyl chloride-hydroxypropyl methacrylate 10 partscopolymer (—SO₃Na group content: 0.7 × 10⁻⁴ eq./g) Polyesterpolyurethaneresin 5 parts (—SO₃Na group content: 1 × 10⁻⁴ eq./g) Methyl ethyl ketone130 parts Toluene 80 parts Myristic acid 1 part Butyl stearate 1.5 partsCyclohexanone 65 parts<Components (1) of a Magnetic Paint>

Yttrium-iron nitrogen magnetic iron metal powder 100 parts (prepared inStep (A) above) Vinyl chloride-hydroxypropyl acrylate copolymer 8 parts(—SO₃Na group content: 0.7 × 10⁻⁴ eq./g) Polyesterpolyurethane resin 4parts (—SO₃Na group content: 1 × 10⁻⁴ eq./g) α-Alumina (av. particlesize: 80 nm) 10 parts Carbon black (av. particle size: 25 nm) 1.5 partMyristic acid 1.5 parts Methyl ethyl ketone 133 parts Toluene 100 parts<Components (2) of a Magnetic Paint>

Stearic acid 1.5 parts  Polyisocyanate (Colonate L manufactured by  4parts Nippon Polyurethane Kogyo K.K.) Cyclohexanone 133 parts  Toluene33 parts

The primer paint was applied on a nonmagnetic support made of apolyethylene naphthalate film having a thickness of 6 μm (coefficientsof thermal shrinkage in the machine and transverse directions: 0.8% and0.6%, respectively when measured at 105° C. for 30 minutes) so that theprimer layer could have a thickness of 2 μm after being dried andcalendered, and then, the above magnetic paint was applied on the primerlayer by a wet-on-wet method so that the magnetic layer could have athickness of 120 nm after being oriented in a magnetic field, dried andcalendered.

Then, a backcoat paint was applied on the back face of the nonmagneticsupport opposite to the surface carrying the primer layer and themagnetic layer so that the backcoat layer could have a thickness of 700nm after being dried and calendered, and then dried.

The backcoat paint was prepared by dispersing the following componentsof a backcoat paint with a sand mill at a residence time of 45 minutes,and then adding a polyisocyanate (8.5 parts), followed by stirring andfiltrating.

<Components of a Backcoat Paint>

Carbon black (av. particle size: 25 nm) 40.5 parts Carbon black (av.particle size: 350 nm) 0.5 part Barium sulfate 4.05 parts Nitrocellulose28 parts Polyurethane resin (containing —SO₃Na groups) 20 partsCyclohexanone 100 parts Toluene 100 parts Methyl ethyl ketone 100 parts

The magnetic sheet obtained was planished by five-stage calendering at atemperature of 70° C. under a linear pressure of 150 kgf/cm, and woundonto a core and aged at 60° C. and 40% RH for 48 hours. The magneticsheet was cut into a plurality of magnetic tapes each having a width of½ inch. Then, the magnetic tape was abraded with a ceramic wheel(revolution: +150%, winding angle: 30 degrees) while running the tape ata rate of 100 m/min to obtain the magnetic tape having a length of 609m. This magnetic tape was set in a cartridge to obtain a tape for acomputer.

Example 2

A magnetic tape was produced in the same manner as in Example 1 exceptthat the thickness of the magnetic layer after the orientation in amagnetic field, drying and calendering was changed to 50 nm in theproduction process of the magnetic tape, and then the tape was set in acartridge to obtain a tape for a computer.

Example 3

In the production process of an iron nitride magnetic powder accordingto Example 1, the magnetic powder, which had been coated with yttriumhydroxide, was redispersed in a solution of 0.07 mole of boric aciddissolved in 30 cc of water, then the dispersion was filtrated and thepowder was dried at 60° C. for 4 hours to remove water. Thereby, amagnetic powder, the particles of which were covered with yttriumcontaining boron, was obtained. This magnetic powder was reduced withhydrogen, nitrided with ammonia and stabilized in the same manner as inExample 1 to obtain a yttrium-iron nitride-boron magnetic powder.

The contents of yttrium and nitrogen of the resulting yttrium-ironnitride-boron magnetic powder were analyzed with a fluorescent X-rayanalysis and 4.0 atomic % and 9.0 atomic %, respectively, based on theamount of iron. The content of boron was 3.0 atomic %. The X-raydiffraction pattern showed the profile corresponding to the Fe₁₆N₂phase.

Furthermore, the particles were observed with a high dissolutiontransmission electron microscope. The particle shape was substantiallyspherical, and the average particle size was 20 nm. The magnetic powderhad a BET specific surface area of 56.1 m²/g.

The saturation magnetization and coercive force of the magnetic powderof this Example, which were measured by applying a magnetic field of1,270 kA/m (16 kOe), were 105.7 Am²/kg (105.7 emu/g) and 213.4 kA/m(2,680 Oe), respectively.

A magnetic tape was produced in the same manner as in the productionprocess of a magnetic tape of Example 1 except that 100 parts of theyttrium-iron nitrogen-boron magnetic powder produced in the previousstep was used in place of 100 parts of the yttrium-iron nitride magneticpowder in Components (1) of a magnetic paint, and then the magnetic tapewas set in a cartridge to obtain a tape for a computer.

Example 4

A magnetic tape was produced in the same manner as in Example 3 exceptthat the thickness of the magnetic layer after the drying andcalendering was changed to 30 nm in the production process of themagnetic tape, and then the tape was set in a cartridge to obtain a tapefor a computer.

Example 5 Preparation of Iron Nitride Magnetic Powder

Magnetite particles having a substantially spherical shape and anaverage particle size of 18 nm (10 g) was dispersed in water (500 cc)with an ultrasonic disperser for 30 minutes. To this dispersion, yttriumnitrate (0.5 g) was added and dissolved followed by stirring for 30minutes.

Separately, sodium hydroxide (0.16 g) was dissolved in water (100 cc).This aqueous solution of sodium hydroxide was dropwise added to theabove dispersion over about 30 minutes. After the addition of theaqueous solution, the mixture was stirred for further 1 hour.

To this dispersion, sodium silicate (1.13 g) was added and dissolvedfollowed by stirring for 30 minutes. To the dispersion containingdissolved sodium silicate, nitric acid (0.6 g) diluted by 10 times wasdropwise added over 30 minutes.

By the above treatments, yttrium hydroxide and silicon hydroxide weredeposited on the surfaces of the magnetite particles. Then, themagnetite particles washed with water and filtrated. The recoveredparticles were dried at 90° C. to obtain a powder comprising themagnetite particles the surfaces of which were covered with yttriumhydroxide and silicon hydroxide.

The powder comprising the magnetite particles the surfaces of which werecovered with yttrium hydroxide and silicon hydroxide was heated andreduced in a hydrogen stream at 430° C. for 2 hours to obtain anyttrium-silicon-iron magnetic powder. This powder was cooled to 150° C.over about 1 hour while flowing the hydrogen gas, and then the hydrogengas was switched to an ammonia gas, and the particles were nitrided for30 hours while maintaining the temperature at 150° C. Thereafter, theparticles were cooled from 150° C. to 90° C. while flowing the ammoniagas, and then the ammonia gas was switched to a mixed gas of oxygen andnitrogen to stabilize the particles for 2 hours.

After that, the particles were further cooled from 90° C. to 40° C. andmaintained at 40° C. for about 10 hours, while flowing the mixed gas ofoxygen and nitrogen, and then they were recovered in an air.

The contents of yttrium, silicon and nitrogen of the resultingyttrium-silicon-iron nitride magnetic powder were analyzed with afluorescent X-ray analysis and 1.1 atomic %, 2.8 atomic % and 10.3atomic %, respectively, based on the amount of iron. The X-raydiffraction pattern showed the profile corresponding to the Fe₁₆N₂phase.

Furthermore, the particles were observed with a high dissolutiontransmission electron microscope. The particle shape was substantiallyspherical, and the average particle size was 15 nm. The magnetic powderhad a BET specific surface area of 90.1 m²/g.

The saturation magnetization and coercive force of the magnetic powderof this Example, which were measured by applying a magnetic field of1,270 kA/m (16 kOe), were 102.8 Am²/kg (102.8 emu/g) and 215.8 kA/m(2,710 Oe), respectively.

<Production of Magnetic Tape>

A magnetic tape was produced in the same manner as in Example 2 exceptthat the thickness of the magnetic layer after the orientation in amagnetic field, drying and calendering was changed to 50 nm in theproduction process of the magnetic tape, and then the tape was set in acartridge to obtain a tape for a computer.

Example 6 Production of Iron Nitride Magnetic Powder

Silicon hydroxide was deposited on the surfaces of magnetite particlesin the same manner as in Example 5 except that no yttrium nitrate wasadded and the amount of sodium silicate added was changed from 1.13 g to2.03 g, and the amount of nitric acid was changed from 0.6 g to 1.08 g.Then, the magnetite particles were washed with water and filtrated, andthe recovered particles were dried at 90° C. to obtain the magnetitepowder the particle surfaces of which were covered with siliconhydroxide.

Thereafter, the powder comprising the magnetite particles the surfacesof which were covered with silicon hydroxide was heated and reduced in ahydrogen stream under the same conditions as those in Example 5 toobtain a silicon-iron magnetic powder. This powder was then nitrided andstabilized under the same conditions as those in Example 5 and recoveredin the air.

The contents of silicon and nitrogen of the resulting silicon-ironnitride magnetic powder were analyzed with a fluorescent X-ray analysisand 4.7 atomic % and 11.2 atomic %, respectively, based on the amount ofiron. The X-ray diffraction pattern showed the profile corresponding tothe Fe₁₆N₂ phase.

Furthermore, the particles were observed with a high dissolutiontransmission electron microscope. The particle shape was substantiallyspherical, and the average particle size was 14 nm. The magnetic powderhad a BET specific surface area of 92.3 m²/g.

The saturation magnetization and coercive force of the magnetic powderof this Example, which were measured by applying a magnetic field of1,270 kA/m (16 kOe), were 98.1 Am²/kg (98.1 emu/g) and 214.1 kA/m (2,690Oe), respectively.

<Production of Magnetic Tape>

A magnetic tape was produced in the same manner as in Example 2 exceptthat the thickness of the magnetic layer after the orientation in amagnetic field, drying and calendering was changed to 50 nm in theproduction process of the magnetic tape, and then the tape was set in acartridge to obtain a tape for a computer.

Example 7 Preparation of Iron Nitride Magnetic Powder

Magnetite particles having a substantially spherical shape and anaverage particle size of 18 nm (10 g) was dispersed in water (500 cc)with an ultrasonic disperser for 30 minutes. To this dispersion, yttriumnitrate (0.5 g) was added and dissolved followed by stirring for 30minutes.

Separately, sodium hydroxide (0.16 g) was dissolved in water (100 cc).This aqueous solution of sodium hydroxide was dropwise added to theabove dispersion over about 30 minutes. After the addition of theaqueous solution, the mixture was stirred for further 1 hour.

To this dispersion, sodium aluminate (0.68 g) was added and dissolvedfollowed by stirring for 30 minutes. To the dispersion containingdissolved sodium aluminate, nitric acid (0.6 g) diluted by 10 times wasdropwise added over 30 minutes.

By the above treatments, yttrium hydroxide and aluminum hydroxide weredeposited on the surfaces of the magnetite particles. Then, themagnetite particles washed with water and filtrated. The recoveredparticles were dried at 90° C. to obtain a powder comprising themagnetite particles the surfaces of which were covered with yttriumhydroxide and aluminum hydroxide.

The powder comprising the magnetite particles the surfaces of which werecovered with yttrium hydroxide and aluminum hydroxide was heated andreduced in a hydrogen stream at 430° C. for 2 hours to obtain anyttrium-aluminum-iron magnetic powder. This powder was cooled to 150° C.over about 1 hour while flowing the hydrogen gas, and then the hydrogengas was switched to an ammonia gas, and the particles were nitrided for30 hours while maintaining the temperature at 150° C. Thereafter, theparticles were cooled from 150° C. to 90° C. while flowing the ammoniagas, and then the ammonia gas was switched to a mixed gas of oxygen andnitrogen to stabilize the particles for 2 hours.

After that, the particles were further cooled from 90° C. to 40° C. andmaintained at 40° C. for about 10 hours, while flowing the mixed gas ofoxygen and nitrogen, and then they were recovered in an air.

The contents of yttrium, aluminum and nitrogen of the resultingyttrium-aluminum-iron nitride magnetic powder were analyzed with afluorescent X-ray analysis and 1.1 atomic %, 3.1 atomic % and 9.8 atomic%, respectively, based on the amount of iron. The X-ray diffractionpattern showed the profile corresponding to the Fe₁₆N₂ phase.

Furthermore, the particles were observed with a high dissolutiontransmission electron microscope. The particle shape was substantiallyspherical, and the average particle size was 15 nm. The magnetic powderhad a BET specific surface area of 93.3 m²/g.

The saturation magnetization and coercive force of the magnetic powderof this Example, which were measured by applying a magnetic field of1,270 kA/m (16 kOe), were 103.1 Am²/kg (103.1 emu/g) and 211.7 kA/m(2,660 Oe), respectively.

<Production of Magnetic Tape>

A magnetic tape was produced in the same manner as in Example 2 exceptthat the thickness of the magnetic layer after the orientation in amagnetic field, drying and calendering was changed to 50 nm in theproduction process of the magnetic tape, and then the tape was set in acartridge to obtain a tape for a computer.

Example 8 Production of Iron Nitride Magnetic Powder

Aluminum hydroxide was deposited on the surfaces of magnetite particlesin the same manner as in Example 7 except that no yttrium nitrate wasadded, the amount of sodium aluminate added was changed from 0.68 g to1.0 g, and the amount of nitric acid was changed from 0.6 g to 0.9 g.Then, the magnetite particles were washed with water and filtrated, andthe recovered particles were dried at 90° C. to obtain the magnetitepowder the particle surfaces of which were covered with aluminumhydroxide.

Thereafter, the powder comprising the magnetite particles the surfacesof which were covered with aluminum hydroxide was heated and reduced ina hydrogen stream under the same conditions as those in Example 5 toobtain an aluminum-iron magnetic powder. This powder was then nitridedand stabilized under the same conditions as those in Example 5 andrecovered in the air.

The contents of aluminum and nitrogen of the resulting aluminum-ironnitride magnetic powder were analyzed with a fluorescent X-ray analysisand 4.6 atomic % and 10.0 atomic %, respectively, based on the amount ofiron. The X-ray diffraction pattern showed the profile corresponding tothe Fe₁₆N₂ phase.

Furthermore, the particles were observed with a high dissolutiontransmission electron microscope. The particle shape was substantiallyspherical, and the average particle size was 15 nm. The magnetic powderhad a BET specific surface area of 90.5 m²/g.

The saturation magnetization and coercive force of the magnetic powderof this Example, which were measured by applying a magnetic field of1,270 kA/m (16 kOe), were 95.6 A/m²/kg (95.6 emu/g) and 214.1 kA/m(2,690 Oe), respectively.

<Production of Magnetic Tape>

A magnetic tape was produced in the same manner as in Example 2 exceptthat the thickness of the magnetic layer after the orientation in amagnetic field, drying and calendering was changed to 50 nm in theproduction process of the magnetic tape, and then the tape was set in acartridge to obtain a tape for a computer.

Example 9 Preparation of Iron Nitride Magnetic Powder

Magnetic powder comprising magnetite particles the surfaces of whichwere covered with yttrium hydroxide and silicon hydroxide was preparedin the same manner as in Example 5 except that magnetite particleshaving a substantially spherical shape and an average particle size of11 nm was used.

The powder comprising the magnetite particles the surfaces of which werecovered with yttrium hydroxide and silicon hydroxide was heated andreduced in a hydrogen stream at 400° C. for 2 hours to obtain anyttrium-silicon-iron magnetic powder. This powder was cooled to 150° C.over about 1 hour while flowing the hydrogen gas, and then the hydrogengas was switched to an ammonia gas, and the particles were nitrided for30 hours while maintaining the temperature at 150° C. Thereafter, theparticles were cooled from 150° C. to 90° C. while flowing the ammoniagas, and then the ammonia gas was switched to a mixed gas of oxygen andnitrogen to stabilize the particles for 2 hours.

After that, the particles were further cooled from 90° C. to 40° C. andmaintained at 40° C. for about 10 hours, while flowing the mixed gas ofoxygen and nitrogen, and then they were recovered in an air.

The contents of yttrium, silicon and nitrogen of the resultingyttrium-silicon-iron nitride magnetic powder were analyzed with afluorescent X-ray analysis and 1.1 atomic %, 2.9 atomic % and 9.3 atomic%, respectively, based on the amount of iron. The X-ray diffractionpattern showed the profile corresponding to the Fe₁₆N₂ phase.

Furthermore, the particles were observed with a high dissolutiontransmission electron microscope. The particle shape was substantiallyspherical, and the average particle size was 9 nm. The magnetic powderhad a BET specific surface area of 153.3 m²/g.

The saturation magnetization and coercive force of the magnetic powderof this Example, which were measured by applying a magnetic field of1,270 kA/m (16 kOe), were 81.1 Am²/kg (81.1 emu/g) and 200.6 kA/m (2,520Oe), respectively.

<Production of Magnetic Tape>

A magnetic tape was produced in the same manner as in Example 2 exceptthat the thickness of the magnetic layer after the orientation in amagnetic field, drying and calendering was changed to 50 nm in theproduction process of the magnetic tape, and then the tape was set in acartridge to obtain a tape for a computer.

Comparative Example 1

Yttrium-iron nitride magnetic powder was prepared in the same manner asin Example 1 except that magnetite powder having an average particlesize of 85 nm was used as a raw material in place of magnetite powderhaving an average particle size of 25 nm.

The contents of yttrium and nitrogen of the resulting yttrium-ironnitride magnetic powder were analyzed with a fluorescent X-ray analysisand 5.0 atomic % and 12.5 atomic %, respectively, based on the amount ofiron. The X-ray diffraction pattern showed the profile corresponding tothe Fe₁₆N₂ phase. Furthermore, the particles were observed with a highdissolution transmission electron microscope. The particle shape wassubstantially spherical, and the average particle size was 60 nm. Themagnetic powder had a BET specific surface area of 8.3 m²/g. Thesaturation magnetization and coercive force of the magnetic powder ofthis Example, which were measured by applying a magnetic field of 1,270kA/m (16 kOe), were 194.2 Am²/kg (194.2 emu/g) and 183.9 kA/m (2,310Oe), respectively.

Then, a magnetic tape was produced in the same manner as in theproduction process of a magnetic tape of Example 1 except that 100 partsof the yttrium-iron nitrogen-boron magnetic powder produced in theprevious step was used in place of 100 parts of the yttrium-iron nitridemagnetic powder in Components (1) of a magnetic paint, and then themagnetic tape was set in a cartridge to obtain a tape for a computer.

Comparative Example 2

A magnetic tape was produced in the same manner as in ComparativeExample 1 except that the thickness of the magnetic layer after dryingand calendering was changed to 90 nm, and then the magnetic tape was setin a cartridge to obtain a tape for a computer.

Comparative Example 3

A magnetic tape was produced in the same manner as in Example 1 exceptthat 100 parts of acicular iron-cobalt alloy magnetic powder [Co/Fe:24.6% by weight; specific surface area: 55.3 m²/g; coercive force: 183.1kA/m (2,300 Oe); saturation magnetization: 135.0 Am²/kg (135.0 emu/g);average major axis length: 80 nm; acicular ratio: 3] was used in placeof 100 parts of the yttrium-iron nitride magnetic powder in Components(1) of a magnetic paint, and then the magnetic tape was set in acartridge to obtain a tape for a computer.

Comparative Example 4 Preparation of Magnetic Powder

Magnetite particles having a substantially spherical shape and anaverage particle size of 25 nm (10 g) was dispersed in water (500 cc)with an ultrasonic disperser for 30 minutes. To this dispersion, yttriumnitrate (2.5 g) was added and dissolved followed by stirring for 30minutes. Separately, sodium hydroxide (0.8 g) was dissolved in water(100 cc). This aqueous solution of sodium hydroxide was dropwise addedto the above dispersion over about 30 minutes. After the addition of theaqueous solution, the mixture was stirred for further 1 hour. Thereby,yttrium hydroxide was deposited on the surfaces of the magnetiteparticles. Then, the magnetite particles washed with water andfiltrated. The recovered particles were dried at 90° C. to obtain apowder comprising the magnetite particles the surfaces of which werecovered with yttrium hydroxide.

The powder comprising the magnetite particles the surfaces of which werecovered with yttrium hydroxide was heated and reduced in a hydrogenstream at 450° C. for 2 hours to obtain an yttrium-iron magnetic powder.This powder was cooled to 90° C., and then the hydrogen gas was switchedto a mixed gas of oxygen and nitrogen to stabilize the particles for 2hours. After that, the particles were further cooled from 90° C. to 40°C. and maintained at 40° C. for about 10 hours, while flowing the mixedgas of oxygen and nitrogen, and then they were recovered in an air.

Thus, the yttrium-iron magnetic powder was produced without nitriding.The content of yttrium in this magnetic powder was 5.4 atomic %. TheX-ray diffraction pattern had a diffraction peak assigned to the α-Fephase.

Furthermore, the particles were observed with a high dissolutiontransmission electron microscope. The particle shape was substantiallyspherical, and the average particle size was 22 nm. The magnetic powderhad a BET specific surface area of 55.1 m²/g.

The saturation magnetization and coercive force of the magnetic powderof this Example, which were measured by applying a magnetic field of1,270 kA/m (16 kOe), were 137.7 Am²/kg (137.7 emu/g) and 156.0 kA/m(1,960 Oe), respectively.

Thereafter, a magnetic tape was produced in the same manner as inExample 1 except that the thickness of the magnetic layer after theorientation in the magnetic field, drying and calendering was changed to120 nm, and then the magnetic tape was set in a cartridge to obtain atape for a computer.

Comparative Example 5 Preparation of Magnetic Powder

Magnetite particles having a substantially spherical shape and anaverage particle size of 25 nm (10 g) was dispersed in water (500 cc)with an ultrasonic disperser for 30 minutes. To this dispersion, yttriumnitrate (2.5 g) was added and dissolved followed by stirring for 30minutes. Separately, sodium hydroxide (0.8 g) was dissolved in water(100 cc). This aqueous solution of sodium hydroxide was dropwise addedto the above dispersion over about 30 minutes. After the addition of theaqueous solution, the mixture was stirred for further 1 hour. Thereby,yttrium hydroxide was deposited on the surfaces of the magnetiteparticles. The magnetic powder obtained was redispersed in a solution of0.007 mole of boric acid dissolved in 30 cc of water, then thedispersion was filtrated and the powder was dried at 60° C. for 4 hoursto remove water. Thereby, a magnetic powder, the particles of which werecovered with yttrium containing boron, was obtained. This magneticpowder washed with water and filtrated, followed by drying at 90° C. toobtain a magnetic powder comprising the magnetite particles the surfacesof which were covered with yttrium hydroxide and boron.

The powder comprising the magnetite particles the surfaces of which werecovered with yttrium hydroxide was heated and reduced in a hydrogenstream at 450° C. for 2 hours to obtain an yttrium-boron-iron magneticpowder. This powder was cooled to 90° C., and then the hydrogen gas wasswitched to a mixed gas of oxygen and nitrogen to stabilize theparticles for 2 hours. After that, the particles were further cooledfrom 90° C. to 40° C. and maintained at 40° C. for about 10 hours, whileflowing the mixed gas of oxygen and nitrogen, and then they wererecovered in an air.

Thus, the yttrium-boron-iron magnetic powder was obtained withoutnitriding.

The contents of yttrium and boron of the resulting yttrium-boron-ironmagnetic powder were analyzed with a fluorescent X-ray analysis and 4.0atomic % and 3.1 atomic %, respectively, based on the amount of iron.The X-ray diffraction pattern showed the diffraction peak assigned tothe α-Fe phase. Thus, it was confirmed that the yttrium-iron nitridemagnetic powder of this Example composed of the mixed phase of theFe₁₆N₂ phase and the α-Fe phase.

Furthermore, the particles were observed with a high dissolutiontransmission electron microscope. The particle shape was substantiallyspherical, and the average particle size was 22 nm. The magnetic powderhad a BET specific surface area of 54.3 m²/g.

The saturation magnetization and coercive force of the magnetic powderof this Example, which were measured by applying a magnetic field of1,270 kA/m (16 kOe), were 133.9 Am²/kg (133.9 emu/g) and 172.7 kA/m(2,170 Oe), respectively.

Thereafter, a magnetic tape was produced in the same manner as inExample 1 except that the thickness of the magnetic layer after theorientation in the magnetic field, drying and calendering was changed to120 nm, and then the magnetic tape was set in a cartridge to obtain atape for a computer.

With the magnetic tapes of Examples 1-9 and Comparative Examples 1-5, acoercive force in the longitudinal direction, a squareness ratio and theproduct of a saturated magnetic flux density and a thickness of amagnetic layer were measured and calculated as magnetic properties ofthe magnetic tapes, and the electromagnetic conversion characteristicwas measured by the method described below. The results are shown inTable 1. This Table also includes the average particle sizes of themagnetic powders used in the production of the magnetic tapes, and thethickness of the magnetic layer.

<Measurement of Electromagnetic Conversion Characteristic>

After running a magnetic tape five times using a LTO drive (manufacturedby Hewlett-Packard) at 40° C. and 5% RH, random data signals having theshortest recording wavelength of 0.33 μm are recorded in the magnetictape and an output was read with a reproduction head, and the output(dB) is reported as a relative value (dB) to that of Comparative Example3 (0 dB as the standard).

TABLE 1 Example No. Comp. Example No. 1 2 3 4 1 2 3 Av. particle 22 2220 20 60 60 60 size of (major magnetic axis powder (nm) length)Thickness 120 50 120 30 120 90 120 of magnetic layer (nm) Coercive 262.7256.4 274.7 267.5 207.0 189.5 195.1 force [Hc] (kAa/m) Squareness 0.890.85 0.91 0.86 0.79 0.72 0.82 ratio [Br/Bm] (Saturated 0.036 0.014 0.0320.008 0.042 0.032 0.043 magnetic flux density) × (thickness of magneticlayer) [Bm · t] (μTm) Output [C] 1.8 2.5 1.9 2.3 0.2 0 0 (dB) C/N (dB)12.5 14.2 8.1 7.9 0.5 0.0 0.8 Example No. Comp. Ex. No. 5 6 7 8 9 4 5Av. particle 15 14 15 15 9 22 22 size of magnetic powder (nm) Thickness50 50 50 50 50 120 120 of magnetic layer (nm) Coercive 261.1 248.4 254.0239.6 234.7 178.1 198.5 force [Hc] (kAa/m) Squareness 0.86 0.85 0.840.83 0.80 0.86 0.89 ratio [Br/Bm] (Saturated 0.012 0.011 0.012 0.0100.009 0.035 0.031 magnetic flux density) × (thickness of magnetic layer)[Bm · t] (μTm) Output [C] 1.9 1.8 1.9 1.7 1.6 0.9 1.1 (dB) C/N (dB) 15.713.1 14.5 12.2 15.0 6.9 6.5

From the results in Table 1, it is understood that the magnetic tapes ofExamples 1-9 had a high output. In particular, when the magnetic layerwas made very thin, for example, 50 nm (Example 2 and 5-9) or 30 nm(Example 4), the properties did no decrease materially, the high outputwas achieved, and the effect to decrease the noise was remarkable. As aresult, the C/N ratio increased, and thus the magnetic tapes had theexcellent high density recording characteristics.

On the contrary, when the substantially spherical magnetic particleshaving a large particle size was used as in the magnetic tapes ofComparative Examples 1 and 2, the surface smoothness of the tapesdeteriorated, and the output greatly decreased when the magnetic layerwas made thin. When the acicular magnetic powder was used as in themagnetic tape of Comparative Example 3, the high output was notobtained, and furthermore the noise level greatly increased incomparison with the magnetic tapes according to the present invention.As a result, the C/N ratio was much smaller than that of the magnetictapes according to the present invention. In addition, with the magneticpowder which had the small particle size but were not nitrided as in thecase of the magnetic tapes of Comparative Examples 4 and 5, the coerciveforce was not so large as that of the magnetic tapes according to thepresent invention. Thus, the output of the magnetic tapes of ComparativeExamples 4 and 5 was lower than the magnetic tapes according to thepresent invention. The noise characteristics were influenced by such asmall coercive force, and the C/N ratio was inferior to that of themagnetic tapes according to the present invention.

EFFECTS OF THE INVENTION

As described above, the present invention uses a magnetic powder, whichcomprises substantially spherical or ellipsoidal particles and at leastiron and nitrogen as constituent elements, and has a Fe₁₆N₂ phase and anaverage particle size of 5 to 30 nm, to form a very thin magnetic layerhaving a thickness of 300 nm or less which does not decrease an output.Therefore, the magnetic recording medium of the present invention has ahigher coercive force and a better surface smoothness than theconventional magnetic recording media. Thereby, the output is furtherincreased, and the magnetic recording medium having excellent shortwavelength recording characteristics can be provided.

1. A magnetic recording medium comprising a nonmagnetic support and amagnetic layer formed on the support and containing a magnetic powderand a binder, wherein said magnetic powder comprises substantiallyspherical or ellipsoidal particles and said particles have an outerlayer containing at least one element selected from the group consistingof rare earth elements, silicon and aluminum, and a core containing aFe₁₆N₂ phase and wherein said particles have an average particle size of5 to 50 nm, wherein said particles have an axis ratio (a ratio of amajor axis to a minor axis) of 1 to 2, and wherein said magnetic layerhas a coercive force of 234.7 to 318.4 kA/m (2,949 to 4,000 Oe).
 2. Themagnetic recording medium according to claim 1, wherein said particleshave an average particle size of 5 to 30 nm.
 3. The magnetic recordingmedium according to claim 1, wherein a content of nitrogen is from 1.0to 20.0 atomic % based on the amount of iron in the magnetic powder. 4.The magnetic recording medium according to claim 1, wherein the rareearth element is at least one element selected from the group consistingof yttrium, samarium and neodymium.
 5. The magnetic recording mediumaccording to claim 1, wherein a content of the rare earth element isfrom 0.05 to 20.0 atomic % based on the amount of iron in the magneticpowder.
 6. The magnetic recording medium according to claim 1, wherein atotal content of silicon and aluminum is from 0.1 to 20.0 atomic % basedon the amount of iron in the magnetic powder.
 7. The magnetic recordingmedium according to claim 1, which has a squareness ratio (Br/Bm) of 0.6to 0.9 both in the longitudinal direction, and a product (Bm·t) of asaturated magnetic flux density (Bm) and a thickness (t) of a magneticlayer of 0.001 to 0.1 μTm.
 8. The magnetic recording medium according toclaim 1, which further comprises a primer layer comprising at least onenonmagnetic powder and a binder, wherein said magnetic layer has athickness of 10 to 300 nm.